Unusual behavior of benzoic acid at low temperature: Raman spectroscopic study

• Raman spectra of benzoic acid single crystals are taken in the temperature range of 5–300 K.
• One of two intra-dimer H-bond stretching modes disappears at T < 60 K due to loss of symmetry.
• The series of weak and narrow bands arises in the high-wavenumber region of 2500–3700 cm−1 at T < 60 K.
• A transition of two wells potential of H-bond to single well potential occurs at low temperature.
• The coupling of intramolecular and O–H stretching vibrations forms states, which resemble soliton states.

The Raman spectra of benzoic acid single crystals have been measured in the temperature range of 5–300 K. At T < 60 K the spectra show at least two anomalous features, one of which is of direct relevance to intensity changes of the lattice modes in the low-wavenumber region. The intensity of modes at ∼86 and ∼146 cm−1 tends to zero at T → 0 K. It is associated with appearance of two H-bonds of different length in the same l-tautomer, and with the loss of the inversion center in the dimer. The modes at ∼86 and ∼146 cm−1 are assigned to symmetric stretching intra-dimer vibrations of the OH⋯O hydrogen bonds of the first and second order, respectively. The assignment is based on the measurements of spectral parameters as function of temperature. The other anomaly is that the series of weak and narrow bands arises in the high-wavenumber region of 2500–3700 cm−1. The bands are assigned to combination tones of O–H hydrogen bonded stretching vibration and intramolecular modes. This effect results from a low-temperature transition of a conventional two wells potential of short H-bond in the l-tautomer to asymmetrical single well potential, and is due to a strong coupling of intramolecular vibrations to O–H stretching.

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